The present invention relates to a liquid ejection head that ejects a droplet by causing an electrostatic force to act on a solution in which charged particles are dispersed, and a method of producing the same.
Nowadays, a thermal-type ink jet head that ejects an ink droplet by means of an expansive force of an air bubble generated in ink through heating of the ink and a piezoelectric-type ink jet head that ejects an ink droplet by giving a pressure to ink using a piezoelectric element have been proposed as liquid ejection heads. In the thermal-type ink jet head, however, the ink is partially heated to 300° C. or higher, and a problem arises in that a material for the ink is limited. Also, when using the piezoelectric-type ink jet head, there occurs a problem in that its structure is complicated and an increase in cost is inevitable.
As a liquid ejection head that solves the problems described above, a system is proposed which uses ink containing a charged fine particle component and controls ejection of the ink by utilizing an electrostatic force through application of a predetermined voltage to each control electrode of the ink jet head in accordance with image data, thereby recording an image corresponding to the image data on a recording medium.
Various ink jet recording apparatuses adopting the electrostatic ink jet recording system are known (see JP 10-230608 A, JP 09-309208 A, JP 10-76664 A, JP 11-105293 A, and JP 08-149253 A, for instance).
FIG. 25 is a conceptual diagram schematically showing an example of an outline construction of an ink jet head of an ink jet recording apparatus disclosed in JP 10-230608 A. This drawing conceptually shows the periphery of one ink guide serving as an ink ejection position of the ink jet head disclosed therein. An ink jet head 400 shown in FIG. 25 includes a head substrate 402, an ink guide 404, an electrical insulating substrate 406, a control electrode 408, a counter electrode 410 supporting a recording medium P, a bias voltage supply 412, and a signal voltage supply 414.
The ink guide 404 has a convex tip end portion 404a including an ink guide groove 420 obtained through cutting by a predetermined width and is arranged on the head substrate 402. Also, in the insulating substrate 406, a through-hole (ejection opening) 418 is established at a position corresponding to arrangement of the ink guide 404. The ink guide 404 passes through the through-hole 418 and protrudes upwardly from a surface of the insulating substrate 406 on a recording medium P side. In addition, the head substrate 402 and the insulating substrate 406 are arranged so as to be spaced apart from each other by a predetermined distance and a gap between these substrates 402 and 406 is defined as a flow path 416 of ink Q.
The control electrode 408 is provided in a ring manner for each through-hole 418 on the surface of the insulating substrate 406 on the recording medium P side so as to surround the periphery of the through-hole 418. Also, the control electrode 408 is connected to the signal voltage supply 414 that generates a pulse voltage in accordance with image data, and the signal voltage supply 414 is grounded through the bias voltage supply 412.
In addition, the counter electrode 410 is arranged so as to be opposed to the tip end portion 404a of the ink guide 404 and is grounded. The recording medium P is arranged on a surface of the counter electrode 410 on an ink guide 404 side. That is, the counter electrode 410 functions as a platen that supports the recording medium P.
At the time of recording, the ink Q containing colorant particles charged to the same polarity as a voltage applied to the control electrode 408 is circulated by an ink circulation mechanism (not shown) in the ink flow path 416 in a direction from the right side to the left side in FIG. 25. Also, a high voltage of 1.5 kV for example is applied to the control electrode 408 by the bias voltage supply 412. At this time, a part of the ink Q in the ink flow path 416 is supplied to the tip end portion 404a while passing through the ink guide groove 420 by a capillary phenomenon, surface tension, surface wetting, or the like.
Here, a DC voltage of 1.5 kV for example is applied to the control electrode 408 from the bias voltage supply 412 as a constant bias. When a pulse voltage of 500 V for example is applied from the signal voltage supply 414 to the control electrode 408 biased to the DC 1.5 kV as a signal voltage corresponding to an image signal, an ink droplet whose main ingredient is the colorant component, flies out from the tip end portion 404a of the ink guide 404, is attracted by the counter electrode 410, and adheres onto the recording medium P, thereby forming a dot of an image.
As a method of producing such an ink jet head, JP 10-230608 A discloses production of the ink guide through plastic molding.
Also, JP 09-309208 A discloses an ink jet head where no ink guide is provided, a meniscus having an approximately hemispherical shape is formed at an ink outflow opening by means of the pressure of ink flowing out from an ink supply path and the surface tension of the ink, and an ink droplet is ejected by utilizing an electrostatic force.
Also, JP 10-76664 A discloses an image forming apparatus that is capable of performing high-speed drawing using a system that includes accommodation means for accommodating a recording liquid obtained by dispersing charged colorant particles in an insulating liquid, an ink flow path which has an opening arranged at a position spaced apart from an image formation target medium by a predetermined distance and in which the recording liquid is circulated, a first electrode provided in the ink flow path, a second electrode that is provided in the ink flow path so as to be opposed to the first electrode and has a tip end that extends until approximately the same height as the opening, supply means for supplying the recording liquid accommodated in the accommodation means to the opening, and voltage application means for applying voltages to the first and second electrodes in accordance with a predetermined image signal to thereby cause the colorant particles in the recording liquid supplied to the vicinity of the opening to gather and cause the gathering colorant particles to be separated and ejected from the insulating liquid for formation of an image on the image formation target medium.
JP 11-105293 A discloses an ink jet head where like in the case of JP 10-76664 A, ink is caused to flow along a protrusion portion that is an ink guide member and a meniscus is formed at a protrusion of the protrusion plate. This protrusion is obtained by molding an alumina-made electrode base and sharpening a tip end thereof through grinding.
Further, in FIG. 12 of JP 08-149253 A, an ejection head is disclosed in which a conical protrusion that is thick at its base portion and narrows as the distance to the tip end thereof decreases is provided and the surfaces of the protrusion and an individual electrode are continuously covered with a conductive substance. Also, in FIG. 17 of this patent document, as a method of producing the conical protrusion, machining of Si or conductive Si with a semiconductor process is disclosed.
By the way, in the ink jet head disclosed in JP 10-230608 A, the ink is caused to move upwardly until a sharply pointed portion by utilizing a capillary phenomenon. Therefore, there is a problem in that ink supply takes a long period of time and ink droplets having a stabilized size and colorant particle concentration cannot be successively ejected at a high ejection frequency.
As described above, it is impossible to increase the ejection frequency of ink droplets. There is a shortcoming in that high-speed drawing cannot be performed.
Also, in the case of the ink jet head disclosed in JP 09-309208 A, the ejection opening needs to have a hole diameter with which clogging will not occur. Therefore, there is a problem in that it is difficult to cause a minute droplet to fly and a high voltage is required to cause droplet flying.
Further, in the case of the ink jet heads disclosed in JP 10-76664 A and JP 11-105293 A, it is difficult to obtain a two-dimensionally arrayed head structure. There is a problem in that ejection portions cannot be arranged at a high density and it is difficult to record a high-quality image at high speed. Still further, when the ink jet head disclosed in JP 11-105293 A is a line head where it is required to form nozzles at a high density, interferences between the nozzles occur and it is impossible to control the diameters of ink droplets. There is also a problem in that it is difficult to record a high-quality image.
Further, in the ink jet head disclosed in JP 08-149253 A, wiring exists in the flow path. Therefore, there is a problem in that electric field interferences occur and it is difficult to control ejection concentrations between channels.